# Homework Help: Exercise about mol

1. Oct 18, 2012

### Lindsayyyy

Hi everybody,

sorry for my sloppy english. I have to translate the task so if you don't understand something just tell me and I try to rephrase it.

1. The problem statement, all variables and given/known data

The exersice is about the "Knallgas" equation. Two reaction euqations are given:

$$2H + O \xrightarrow \ 2H_2O +\Delta E$$
and
$$2H_{2} + O_{2} \xrightarrow \ 2H_{2}O +2\Delta E$$

The energy which is released in this equation is: dE=285.8kJ

a) Decide which reaction equation is the right one. The decision should be based on experimental obseravtions.

b) How much grams oxygen is needed for an optimal reaction when I have 0.0038 cm³ hydrogen?

c) How much energy is released? How long could someone use that energy to run a hairdryer with it (P=1,2 kW)

2. Relevant equations

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3. The attempt at a solution

a) Well, I think I can discuss that problem using the theorem from Avogrado. The ratio is the same in both equations. But in my opinion the task is missing a value for the volume of the water. But I say that the 2nd equation is right (That's what someone would find in the experiment)

b) I searched for the density of H2

$$\rho \cdot V=m$$

I get m=3.42*10^-4 kg
also
$$\frac {m(H_2)} {m(O_2)}= \frac 1 8$$

Now I just have to multiply the mass of H2 times 8 in order to get the mass from O2. I get m(O2)=2.73*10^-3kg

c) I use the law of conservation of mass. The whole mass of water equals m(H2)+m(O2). I searched for the molar mass of water in order to calculate how much mol I have. Afterwards I calculate the energy the following way:

E= E(given)*mol

and for the last task I used the formula: P=dE/dt

Is my solution right? Are there other possibilities to solve it? I'm uncertain because I had to look up some stuff like molecular weight, density etc. But I don't know if there's another way